It is 3am in the African bush. At an hour when most average tourists would expect to be sleeping soundly - or partying the night away - we shuffle blearily from our tents below Kivuko Rocks and set off once more into the darkness.

Ahead of us are five hours being thrashed and bumped along the dirt tracks of Taita/Rukinga Wildlife Conservancy.

Our task is to monitor and classify the diversity of life in this pristine corner of true African wilderness. Our hope is contact - however fleeting - with one of the enigmas of nature, the maneless lions of Tsavo.

For many, the very words Tsavo lions inspire fear and awe. In 1898 two males - known as Ghost and Darkness - killed and ate dozens, maybe hundreds, of construction engineers building the Tsavo River railway bridge.

But their infamy stretches beyond their ferocity. Unlike all other lions, the males are distinguished by their lack of any significant mane on the neck and shoulders.

The basis of the project, sponsored by Earthwatch, is to determine the cause of manelessness and whether this has any bearing on their aggressive behaviour.

Lion texts

Long term, the aim is to achieve a living balance between lion, the local population and the huge diversity of species that make up Kenya's largest and most secure wildlife sanctuary.

As we venture out into the night we split into two teams, climb into our Land Rovers and begin to sort out our equipment. Loann and Shirley, from Arizona, take the PDA and clipboard to start recording the animals and birds we will see between now and 8 am.

A strong torch picks out the eyes of those animals on the night prowl

Shay from San Francisco sweeps the surrounding scrub with a million candlepower spotlight, looking for the eye-shine of the 100 or more animals that roam the 45,000 hectares (110,000 acre) of savannah.

On the roof, bundled up against the cold and wind, Shay's mother, Lesley, and I begin the ritual of tracking the lions.

Two males, Kabochi and Elton, and one female, Diana, have been collared by Kenyan vets and are now transmitting both RF and GSM signals.

A small Sim card relays their location by sending out an SMS text every four hours; although the scarcity of receivers in the area means that the system is somewhat hit-and-miss.

We are more interested in picking up the RF signals, which we should be able to catch when the lions are within a kilometre of our vehicle. Holding on to the Land Rover with one hand, I rotate the six-pronged aerial in two loops over my head while Lesley tunes in the receiver to the three frequencies on which the lions are, unwittingly, broadcasting.

Sunrise over Tsavo: This is the lions' domain

Through the night, every 5 minutes, we twirl the antenna and listen out for the beeps that mean our prey is close by.

Up front, the spotting and recording are in full swing.

"Two bush babies at 30m. Resting."

"Six zebras at 70m. Walking."

"Two dik-diks at 10m. Walking."

It is repetitive, for some even boring, work. But the data we and the dozen or so other groups each year collect is vital to our understanding of how lions interact with the species around them.

Led by Dr Bruce Patterson, of the Field Museum Chicago, US, scientists are trying to find out if the varying numbers of lions, their prey and rival predators form a pattern. They also want to find out why the lions lack manes.

The Earthwatch programme uses volunteers to collect data

Theories about the cause of this vary. One suggests it is an adaptation to the dense, prickly shrub that characterises the Tsavo region, and makes it easier for the animals to move about.

Another looks to hormone differences between the Tsavo lions and their maned counterparts on the plains.

Certainly, Tsavo's lions are reputed to be larger, more aggressive, and more prone to attack people and livestock than their relatives. Some have even suggested that they are an entirely different kind of lion although this claim has not been substantiated.

It is 5.15, the coldest hour of the night before dawn brings some light and heat. Our arms are beginning to ache and eyelids droop when the two-way radio crackles into life.

It is Alex Gombe, Earthwatch principal investigator, in the other Land Rover. His team has picked up a faint signal from Kabochi and they are trying to locate him in the bush.

Night vigil

We climb down from the roof and hurtle across to join them. As we arrive, someone glimpses a movement in the undergrowth to our left. Then, lit only by our spotlight and the moon, we are suddenly following Kabochi and his companion, Bahati, slowly up the dirt track.

It is a moment of spine-tingling exhilaration. All thoughts of cold or discomfort are banished as we struggle to restrain our feelings in muffled whispers.

Over the next hour, recording their every movement, we trail the two maneless lions up the gentle slope and through dense undergrowth and shrub. At one point, obscured from the lions by the vehicle, Alex jumps out to collect a newly deposited sample of faeces.

Alex Gombe sweeps the area for the collared lions

Its DNA will be analysed at a later date in Chicago. Kabochi and Bahati are thought to be related, perhaps brothers. Each sighting of them together adds incrementally to the jigsaw that is being formed of the overall activities of the two major prides that are thought to prowl the sanctuary.

As the first rays of the sun begin to creep over Mount Kasigau, the bush thickens beyond the endurance of our Land Rovers and we lose sight of the two lions. We continue for two hours more to count and record the dik-diks and zebras, but our minds are still reeling from the hour we spent with Kabochi and Bahati.

Over the next few days we are to encounter eight more lions, including a previously unrecorded mother and two cubs.

Lions are mainly nocturnal and we soon live by their clocks. We follow them as they drink, hunt, eat and even mate. Each sighting is unique and the thrill never diminishes.

At 8.30 am we crash out in our tents as the sun begins to warm the Kivuko Rocks above us. I drift off into fitful sleep, prowling in my dreams through the scrub of Tsavo with the magnificent beasts we have been privileged to follow.